A pumping system includes a pump, a motor and a motor controller. Motor controllers enable effective control of pumps responsive to changing system conditions. These systems can be used to pump fluids from deep wells such as coal bed methane (CBM) and coal seam gas (CSG) wells. Referring to FIG. 1., a basic CBM/CSG vertical well includes tubing run below the deepest coal interval. After fracture stimulation, if required, water flows from the coal seam, travels down through the annulus and is pumped out through the tubing. Methane—desorbed or liberated from the coal matrix—flows into the annulus between the casing and the tubing and rises to the surface where it is piped to a compressor station and combined with production from other wells. Water may be produced as well, which can either be reinjected into a deeper formation or treated and disposed. An electrical submersible progressing cavity pump may be used.
The components of an exemplary CBM/CSG pumping installation are shown in FIG. 2. From the bottom up, the installation includes an electric motor close coupled to a pump. These components are centralized with the aid of centralizers, strategically placed along the total installed length. The motor is powered by a motor lead fed from the surface. Power to the motor is obtained from a motor controller. Water is transported to the surface along the installed drop pipe whilst gas travels up along the casing or annulus (space between the casing and the drop pipe. Water and gas are ejected at surface and collected via surface piping to a central point. A transducer is shown hanging from a transducer cable. Exemplary transducers include pressure and level transducers and/or sensors. The transducer may detect the level of the water in the well or may detect other variables operable to determine the water level. The pumping station is operated to deliquify the well while maintaining adequate suction pressure and backpressure requirements of the well. Managing bottom-hole pressure and deliquification of the well can significantly increase productivity.
Dry-run or pump-off conditions and entrained gasses are harmful and difficult conditions to guard against since CBM/CSG applications require pumping wells down to levels close to the pump intake. This is done to reduce bottom-hole pressure as much as possible. Several conditions make the detection of dry-running difficult, including entrained gas in the water and annulus pressure that may be variable and substantially of the same order of magnitude as the water column pressure. The complexity of determining the water level at levels close to the coal seams or sources of gas that cause increased levels of entrained gas makes it difficult to avoid dry-run or pump-off conditions when relying on bottom-hole pressure measurements. While the pump setting is often below the gas producing zones to avoid gas entering the pump, this is not always possible where pumps are installed in horizontal wells or where gas production levels exceed original expectations.
One of the main concerns with CBM applications is a scenario in which the pump runs for extended time periods either dry or with high free gas production through the pump. This generally results in premature pump failure. Free gas may enter the pump when the fluid level is drawn down to the pump intake, which may occur when the pumping rate exceeds the in-flow rate. Using transducers to determine suction pressure, shaft torque and low water levels is not always possible or practical. Models can be used to determine operating conditions, but changing operating conditions may cause pump characteristics to vary. One example of changing characteristics is pump wear caused by solids in the water. Changes in the liquid, such as gas and particle content, density, debris and other factors make the use of transducers a challenge.
There is a need to provide improved controls to manage pumping systems, particularly in difficult or hard to reach environments.